1 Answer
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Took me a little while to figure this one out. The issue is related to the fact that you are in sandbox mode, but disabling it is not the way to fix this.

When you add a Facebook user to your app (as a tester, developer, admin, etc) they are initially given a Pending status. While the user has that pending status, they can view your Facebook app but can not grant privileges to it.

To remove the pending status, you need the person who you've granted the role to to log into their account. In their notifications you will see that they have been granted the role for the Facebook app. The user then has to click on that notification and accept it. Once accepted, the user is no longer pending and can accept Facebook privilege requests from the App.